Such a method is known from the article of S. Adachi et al entitled "Chemical Etching of InGaAsP/InP DH Wafer", published in Journal of Electrochemical Society, Vol. 129, No. 5, May 1982, pp. 1053-1062. In this article, a method is described of manufacturing mesas in a semiconductor layer structure comprising a comparatively thin first semiconductor layer of InGaAsP between two semiconductor layers of InP, the upper layer of which constitutes the second semiconductor layer. In this case, the semiconductor layers are removed by selective or non-selective wet-chemical etchants, such as HCl and mixtures thereof with H.sub.2 O, H.sub.2 O.sub.2, CH.sub.3 COOH or H.sub.3 PO.sub.4, which are selective etchants for InP, mixtures of H.sub.2 SO.sub.4, H.sub.2 O.sub.2 and H.sub.2 O, which are selective etchants for InGaAsP, and mixtures of HCl and CH.sub.3 COOH or H.sub.3 PO.sub.4 to which H.sub.2 O.sub.2 is added, or mixtures of Br.sub.2 and CH.sub.3 OH, which are non-selective etchants, by which therefore both InP and InGaAsP can be etched. When etching a mesa, use is made of a mask layer comprising SiO.sub.2.
As has been found in practice, a disadvantage of the known method is that with a number of the etchants used therein, especially the non-selective etchants, such as mixtures of Br.sub.2 and CH.sub.3 OH, the known method results in under-etching with respect to the mask. As a result, an accurate definition of the width of the mesa at the area of the first semiconductor layer is impeded because in this case this width depends upon the thickness of the second semiconductor layer. Due to this inaccurate width definition, a number of important properties of, for example, radiation guides or semiconductor diode lasers of which the mesa forms part are undetermined, which is undesirable. Such properties are, for example, the lateral mode in which the radiation is situated and the starting current of a semiconductor diode laser. With some other non-selective etchants, such as mixtures of HCl and HNO.sub.3, the problem sometimes arises that they do not or substantially do not etch InP. If in the known method solely selective etchants are used, another problem arises, which is connected with the fact that the first semiconductor layer is comparatively thin with respect to the second semiconductor layer. It is difficult to accurately etch away this comparatively thin layer because the etchant is a wet-chemical etchant whose etching rate depends upon a large number of factors, such as the accurate composition of the semiconductor layer and the etchant, the temperature of the etchant etc. If the first semiconductor layer must be effectively etched away, that is to say that outside the mesa nothing is left of this layer at any place, some under-etching must be accepted. Due to the under-etching and especially due to the uncertainty with respect to the extent of this underetching, the width of the mesa at the area of the first semiconductor layer becomes uncertain. As a result, the same disadvantages are obtained as mentioned above for etching with non-selective etchants. If in the known method the semiconductor layers are removed alternately by a known chemical etchant etching selectively and by a known chemical etchant etching non-selectively, the aforementioned problems still arise, though to a smaller extent. In parctice, mesas are often desired whose sidewalls are substantially flat, i.e. substantially not curved. For example, the width of a mesa in the direction of height of the mesa varies in an accurately defined manner, as a result of which the aforementioned problems of an undefined width at the area of the first semiconductor layer are smaller. More particularly, these problems substantially do not arise if the mesa has substantially straight sidewalls. Moreover, such flat sidewalls are often very suitable if, for example, by means of the OMVPE growing technique other semiconductor layers are provided beside the mesa. These flat sidewalls are often obtained by utilizing preferential etchants. An additional disadvantage of the known method in this case arises if several etchants are used for etching the mesa and one of these etchants does not etch preferentially or does not etch preferentially in the same manner as one of the other etchants. For example, it has been found that, if the second semiconductor layer comprises InP and the first semiconductor layer comprises InGaAsP and the second semiconductor layer is etched by means of a mixture of HCl and H.sub.3 PP.sub.4, a mesa is obtained having substantially flat and straight sidewalls at the area of the layer. If subsequently the first semiconductor layer is etched by means of an etchant comprising a mixture of Br.sub.2 and CH.sub.3 OH, in spite of the comparatively small thickness of the layer to be etched, the shape of the mesa is attacked due to the fact that also the second semiconductor layer is etched in such a manner that the flatness of the side edge of the mesa at the area of the layer disappears. At the same time, some under-etching occurs with respect to the mask. Finally, the disadvantage has to be mentioned that several chemical etchants give rise to a large under-etching at the area of the interface between the mask and the second semiconductor layer.